Caroline Flint, the shadow energy secretary, has responded to polls suggesting that six out of 10 people dislike Ed Miliband by claiming that the personal popularity of a leader is not the key to winning elections.

Flint said Miliband would be "the first to say we need to redouble our efforts to win the confidence of the public", with two years before the general election. But, after the dire personal polling for the Labour leader, she said that there were historical precedents for the election of unpopular leaders.

"Individual popularity poll ratings are always given prominence, but the truth is that, when it comes to the election, that's not always a significant factor. Think back to Labour leaders in the past who were popular but couldn't win elections. Margaret Thatcher was unpopular but won elections. Sometimes these things are overplayed."

In a wide-ranging interview with the Observer, Flint said that one of the government's most significant failures, when it came to the cost of living, was its inability to deal with the big six energy companies that had made profits of £3.3bn since 2010. At the same time, consumer bills have risen by more than £300 a year. Earlier this summer, the chief executive of one supplier, First Utility, said energy bills were rising so steeply that they could overtake mortgage repayments in parts of Britain in just five years.

Flint said she was concerned that the energy market was not competitive, and there was not enough known about how companies calculated consumer bills. The big six companies – British Gas, which is owned by Centrica, EDF Energy, RWE npower, SSE, E.ON and Scottish Power – generate electricity and tend to sell most of it to their own retail businesses or between themselves.

The coalition is planning to allow generating companies to sign agreements, known as "contracts with a difference" (CfD), where a price for wholesale low-carbon energy is set, irrespective of erratic changes in the market price, so that their risk in investing is reduced.

But Flint said that the energy industry was so opaque that it would be impossible to establish the right set price. There was a danger, she said, that consumers would again lose out if the market price of low-carbon energy ended up being consistently below the price set by government, with the additional costs being passed to the consumer.

"The big six energy companies generate energy, sell it to themselves and sell it to us. We don't really know anything about how that works. This goes to the heart of the public having faith in the price of energy," she said.

"The market at the moment is open to manipulation and price fixing because so much of it is secret." Labour wants the energy generators to pool their output. A new regulator would then monitor the price sold by this pool to the retailers, and the price they then pass on to the consumers.

"We have a system that no one is confident about. It is a travesty that we have had two energy bills that haven't addressed reform of the energy market in any way. Even without CfDs, it would be absolutely right to reform this market and put a premium on openness and transparency, and reassure the public that the price they pay for energy is fair. But also to give a regulator the power to ensure that when wholesale prices drop that they are passed on to the consumer."

Flint also attacked the government's decision to give tax breaks to companies exploring the potential for hydraulic fracturing to produce shale gas, a process known as fracking. She said that the government could not know whether fracking held any potential in the UK and described its championing of the process as "gimmicky". "I can't think of any other area of policy where we are offering tax breaks on the basis of no information."